This invention relates to supported metallocene olefin polymerization catalyst systems, methods of producing the catalysts and methods of polymerizing alpha-olefins with the catalysts to form polymers having narrow molecular weight distribution. More particularly, this invention relates to the preparation of a catalyst precursor component comprising a metallocene containing transition metal compound, a magnesium containing compound, an aluminum compound, a silicon compound and a polymeric support material. The catalyst precursor component is combined with a cocatalyst and used in olefin polymerization.
The polymerization of olefins, such as ethylene, propylene and other alpha-olefins is an important industrial process which is run on a huge scale around the world. Important factors in the polymerization process are the properties and cost of the polymer product which relate to the catalyst used in the process. The use of certain types of catalysts results in the presence of residue of catalyst components in the polymer product. In one type of polymerization process, the catalyst is prepared in situ. In other types of polymerization processes, the catalyst is fully or partially prepared before use in the polymerization process. A catalyst and its components (co-catalyst, modifiers, external electron donors, etc.) can be attached to a solid support material or the catalyst and its components can be added individually. This means that in certain types of catalyst systems, the catalyst components (co-catalyst, modifiers, external electron donors, etc.,) can be added individually to the reaction medium during the polymerization process. On the other hand, there are other cases in which the catalyst can be premixed with all its components during the catalyst preparation process resulting in the addition of one catalyst component to the reaction medium during the polymerization process.
The preparation of supported metallocene catalysts has previously been complicated and expensive. For example, the preparation of supported metallocene catalysts has required expensive and troublesome silica dehydration and aluminoxanes treatment during catalyst preparation. Procedures typically used for the preparation of silica supports such as spray drying or re-crystallization processes are complicated and expensive. Also, high calcination temperatures are required to remove water, which is a common catalyst poison. These steps represent a significant proportion of the preparation of the catalyst. In addition, the use of silica as a support results in the presence of support residue in the polymerization product, which can affect product processing and properties, such as optical properties.
Since the type of catalyst system used plays a major role in the polymerization process, the form and the physical properties of the catalyst system are important. For example, a solid catalyst system in the form of granules, often supported on silica or magnesium chloride, is usually used in gas phase and liquid slurry polymerization processes in order to reduce possible fouling of the reactor.
One of the objects of the present invention is to overcome the difficulties encountered in the prior art. The present invention provides a new development in olefin polymerizations. The supported metallocene catalyst system of the invention in which polyvinylchloride is used as a polymeric support can be used in both slurry and gas phase polymerization processes.
The present invention provides a catalyst composition comprising a catalyst component which comprises a metallocene compound, a magnesium compound, a silicon compound, a hydroxyl containing compound, an aluminum compound and a polymeric material. The silicon compound and/or aluminum compound are optional. The catalyst component, when used in conjunction with a co-catalyst such as an organoaluminum compound or a mixture of organoaluminum compounds, is useful for polymerization of ethylene to linear low and medium density polyethylenes and copolymerization of ethylene with alpha-olefins having about 3 to 18 carbon atoms. The catalyst composition has the ability to produce polymers with narrow molecular weight distributions.
The catalyst composition of the present invention contains a solid catalyst component. The solid catalyst component contains at least a metallocene compound, a magnesium compound, a hydroxyl containing compound, and a polymeric material having a mean particle diameter of 5 xcexcm to 1000 xcexcm, a pore volume of 0.1 cc/g or above, a pore diameter of at least 10 angstroms or a pore diameter of 500 to 10,000 Angstroms and a surface area of from 0.2 m2/gm to 15 m2/gm. The solid catalyst component may also contain a silicon compound and an aluminum compound. The solid catalyst component is useful in olefin polymerization catalysts.
Metallocenes suitable for use in the invention can be represented by the general formula (Cp)zMRwXy wherein Cp represents a substituted or unsubstituted cyclopentadienyl ring; M represents a Group IVB or VB transition metal of the Periodic Table of the Elements (CAS Version); R represents a hydrocarbyl radical such as alkyl, said hydrocarbyl radical containing 1 to 20 carbon atoms, e.g., methyl, ethyl or propyl; X represents a halogen atom; and 1xe2x89xa6zxe2x89xa63, 0xe2x89xa6wxe2x89xa63, and 0xe2x89xa6yxe2x89xa63. The cyclopentadienyl ring may be substituted with a hydrocarbyl radical such as alkyl, alkenyl or aryl, said hydrocarbyl containing 1 to 20 carbon atoms; such as methyl, ethyl, propyl, aryl, isoamyl, isobutyl, phenyl and the like. The preferred transition metals are titanium, zirconium or vanadium.
Preferred metallocene compounds include bis(cyclopentadienyl) zirconium dimethyl, bis(cyclopentadienyl)zirconium methyl chloride, bis(cyclopentadienyl) zirconium ethyl chloride and bis(cyclopentadienyl)zirconium dichloride.
The magnesium compounds used for the solid catalyst component include Grignard compounds represented by the general formula R3MgX, wherein R3 is a hydrocarbyl group of 1 to 20 carbon atoms and X is a halogen atom; preferably chlorine.
The magnesium compound is preferably a reagent with the chemical formula RaMgX2-a wherein R is an alkyl group having 1 to 20 carbon atoms and X is halogen or alkyl group independently having 1 to 20 carbon atoms and a is 0, 1 or 2.
Other preferred magnesium compounds are represented by the general formula R4R5Mg, wherein R4 and R5 are the same or different hydrocarbyl group of 1 to 20 carbon atoms.
Preferred magnesium compounds include the following: dialkylmagnesium such as diethylmagnesium, dipropylmagnesium, di-iso-propylmagnesium, di-n-butylmagnesium, di-iso-butylmagnesium, butylethylmagnesium, dihexylmagnesium, dioctylmagnesium butyloctyl magnesium; alkyl magnesium chloride such as ethylmagnesium chloride, butylmagnesium chloride, hexylmagnesium chloride and the like.
A silicon compound which can be used in the synthesis of the solid catalyst component in this invention can be represented by the general formula MX4, wherein M is a metalloid of Group IV, preferably silicon; and X is a halogen atom, preferably chlorine.
The hydroxyl containing compound used in the synthesis of the solid catalyst component in this invention can be represented by the general formula Z+OHxe2x88x92 or R+OHxe2x88x92 wherein Z+ is a metal of Groups IA to VIA, or a non metal cation, e.g., +NH4 or even H+. R can be a hydrocarbyl group of 1 to 20 carbon atoms, and R is preferably alkyl.
An aluminum compound can be used in small amounts in the synthesis of the solid catalyst component of this invention to treat the polymeric support after treatment with hydroxyl compound. Alkyl aluminoxanes are the preferred aluminum compounds for this treatment of the support.
The polymer particles used in the present invention have a spherical shape with a particle diameter of 5 to 1000 xcexcm, preferably 10 to 800 xcexcm, and more preferably 15 to 500 xcexcm, a pore diameter of 500 to 10,000 Angstroms, a pore volume of 0.1 cm3/g or above, preferably 0.2 cm3/g or above, and a surface area of from 0.2 m2/gm to 15 m2/gm. Examples of the polymeric support useful herein include particles of polyolefins, polyvinylchloride, polyvinylalcohol or polycarbonate, more particularly, beads of polymers such as polyinylchloride, polyvinylalcohol, ethylene-vinylalcohol copolymer, polymethylacrylate, polyethylacrylate, polymethylmethacrylate, and the like. Among these polymeric materials the vinylpolymers are more preferred and polyvinylchloride is most preferred. The preferred shape for the particles is spherical. The polyvinylchloride particles preferably have a molecular weight in the range of 5,000 to 200,000 g/mol.
The polymer particles used in the present invention have surface active sites such as labile chlorine atoms present in polyvinylchloride. Preferably, these active sites are reacted stoichiometrically with the organic magnesium compound.
The use of the polymer particles in this invention offers significant advantages over traditional olefin polymerization catalysts which use supports such as silica or magnesium chloride. In comparison to the silica supported catalyst, the polymer particles described in catalyst preparation of the invention require no high temperature and prolonged dehydration steps prior to their use in catalyst synthesis, thereby simplifying the synthesis process and thus reducing the overall cost of catalyst preparation. Furthermore, the cost of the polymeric support used in the present invention is substantially lower than the cost of silica or magnesium chloride supports. In addition, the catalyst in the present invention uses significantly lower levels of catalyst precursors for catalyst preparation than silica or magnesium chloride supported catalysts. It should be noted that it is both helpful and necessary to reduce the amounts of the catalyst precursors used, since this will help in reducing the catalyst preparation cost and will also help in reducing the amount of chemicals wasted during each catalyst preparation process. For example, in preparing a standard supported metallocene catalyst using silica, a metal loading of about 0.5% is required to produce an active catalyst. Meanwhile, according to the present invention, a metal loading as low as 0.06% is more than enough to produce a highly active catalyst (see Examples 11 and 12 below). This lower metal loading uses an amount of metal precursor which is eight times less than that of the standard supported metallocene catalysts. Also, the catalyst in the present invention is more active than the conventional silica or magnesium chloride supported Ziegler-Nata and conventional metallocene catalyst systems.
According to one embodiment, a polyvinyl chloride support is used. The synthesis of the solid catalyst component in the present invention involves introducing the polymeric material described above into a vessel and then adding a diluent. Suitable diluents include alkanes such as isopentane, hexane, and heptane, and ethers such as diethylether and dibutylether. The polymeric material is then treated with a magnesium compound described above at a temperature in the range of about 20xc2x0 C. to 110xc2x0 C. The ratio of magnesium compound to the polymer support can be in the range of 0.1 mmol to 10 mmol magnesium compound per gram polymer. The excess or unreacted magnesium chloride is then removed by washing with suitable solvents such as hexane, heptane or isooctane.
The resulting free flowing solid product is then slurried. Suitable solvents for slurrying include hexane, cyclohexane, heptane, isooctane and pentamethylheptane. The slurried material is treated with a chlorinating agent such as silicon tetrachloride at a temperature in the range of about 40xc2x0 C. to 120xc2x0 C.
The chlorinated product is further treated with a hydroxyl containing compound such as ammonium hydroxide, sodium hydroxide, ethyl alcohol or even water to hydroxylate the support. The hydroxylated product is washed thoroughly with an organic solvent such as n-hexane. An organoaluminium compound such as methylaluminoxane can be used in small amounts to finally treat the solid support prior to the addition of the metallocene precursor. By metallocene precursor is meant any type of metallocene compound used in the catalyst preparation process. These include, e.g., zirconocene dichloride, methyl zirconocene, aryl zirconocene, etc.
According to this invention, the washed product is optionally treated with organoaluminum compound and then treated with a metallocene compound described above at a temperature in the range of about 40xc2x0 C. to 120xc2x0 C. Bis(cyclopentadienyl)zirconium dichloride is the preferred metallocene compound. The metallocene treated solid catalyst component is then washed with a suitable solvent such isopentane, n-hexane, cyclohexane, n-heptane, isooctane or pentamethylheptane, preferably isopentane or n-hexane, and dried using a nitrogen purge at a temperature in the range of about 20xc2x0 C. to 80xc2x0 C.
In some embodiments in which zirconium containing metallocene and aluminum compounds are used in the synthesis, the final solid catalyst components have a molar ratio of Al:Zr of about 2:1 to about 50:1, preferably about 5:1 to about 20:1.
The catalyst component is activated with suitable activators, also known as co-catalysts, for olefin polymerization. The preferred compounds for activation of the solid catalyst component are organoaluminum compounds.
The cocatalyst is a transition metal (zirconium) site activator, such as aluminoxanes, represented by the general formula 
for a linear aluminoxane, where q represents a number satisfying 0xe2x89xa6qxe2x89xa650 and/or
(Al(R10)xe2x80x94O)s
for a cyclic aluminoxane, wherein s represents a number satisfying 3xe2x89xa6sxe2x89xa650, and wherein R7, R8, R9 and R10 are either the same or different linear, branched or cyclic alkyl group of 1 to 12 carbons; such as methyl, ethyl, propyl or isobutyl. A preferred cocatalyst is a mixture of a trialkyl aluminum and an alkyl aluminoxane. A preferred activator for zirconium sites is methylaluminoxane. Since commercially produced methylaluminoxane contains trimethylaluminum, commercially produced methylaluminoxane can itself be conveniently used to provide a mixture of aluminoxanes.
The organoaluminum compounds in this invention can be used preferably in the range of about 1 to 1500 moles of aluminum per one mole of transition metal in the catalyst, and more preferably in the range of about 50 to 800 moles aluminum per one mole of transition metal. Preferably, the cocatalyst is present in an amount which provides a ratio of aluminum atoms in the cocatalyst to zirconium atoms in the catalyst precursor component from about 10:1 to about 1500:1.
Gel permeation chromatography of polymers produced using the catalyst compositions of this invention shows a narrow molecular weight distribution. The catalyst systems of the present invention are of high productivity of at least about 3-4 kilograms of polymer per gram of catalyst.
The linear polyethylene polymers prepared using the catalyst systems of this invention include homopolymers of ethylene or copolymers of ethylene with one or more C3 to C10 alpha-olefins. Particular examples of these copolymers include ethylene/1-butene copolymers, ethylene/1-hexene copolymers, ethylene/1-octene copolymers, ethylene 4-methyl-1-pentene copolymers. Ethylene/1-hexene and ethylene/1-butene are the most preferred copolymers polymerized using the catalyst systems of this invention.